TPS53353 datasheet(21/29 Pages) TI1

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(

)

(

)

W =

OUT

OCP

DS(on)

TRIP

)

( A)

OUT

INJ _ SW

(

)

(

)

IND ripple

INJ _ OUT

IND ripple

OUT

ESR

INJ _ SW

INJ _ OUT

VFB

0.6

OUT

VFB

TPS53353

www.ti.com

SLUSAK2

– AUGUST 2011

6. CHOOSE THE OVERCURRENT SETTING RESISTOR

The overcurrent setting resistor, RTRIP, can be determined by Equation 10.

where

•

ITRIP is the TRIP pin sourcing current (10 µA)

•

RDS(on) is the thermally compensated on-time resistance value of the low-side MOSFET

(10)

Use an RDS(on) value of 1.6 mΩ for an overcurrent level of approximately 20 A. Use an RDS(on) value of

1.7 m

Ω for overcurrent level of approximately 10 A.

External Component Selection Using All Ceramic Output Capacitors

When a ceramic output capacitor is used, the stability criteria in Equation 5 cannot be satisfied. The ripple

injection approach as shown in Figure 2 is implemented to increase the ripple on the VFB pin and make the

system stable. In addition to the selections made using steps 1 through step 6 in the External Component

Selection section, the ripple injection components must be selected. The C2 value can be fixed at 1 nF. The

value of C1 can be selected between 10 nF to 200 nF.

where

•

N is the coefficient to account for L and COUT variation

(11)

N is also used to provide enough margin for stability. It is recommended N=2 for VOUT ≤ 1.8 V and N=4 for VOUT

≥ 3.3 V or when L ≤ 250 nH. The higher VOUT needs a higher N value because the effective output capacitance is

reduced significantly with higher DC bias. For example, a 6.3-V, 22-

µF ceramic capacitor may have only 8 µF of

effective capacitance when biased at 5 V.

Because the VFB pin voltage is regulated at the valley, the increased ripple on the VFB pin causes the increase

of the VFB DC value. The AC ripple coupled to the VFB pin has two components, one coupled from SW node

and the other coupled from the VOUT pin and they can be calculated using Equation 12 and Equation 13 when

neglecting the output voltage ripple caused by equivalent series inductance (ESL).

(12)

(13)

It is recommended that VINJ_SW to be less than 50 mV. If the calculated VINJ_SW is higher than 50 mV, then other

parameters need to be adjusted to reduce it. For example, COUT can be increased to satisfy Equation 11 with a

higher R7 value, thereby reducing VINJ_SW.

The DC voltage at the VFB pin can be calculated by Equation 14:

(14)

And the resistor divider value can be determined by Equation 15:

(15)

TPS53353 Datasheet(PDF) 21 Page - Texas Instruments

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